This invention relates to a marine propulsion system, and more particularly to an inboard/outboard stern drive system.
In a marine drive system, it is generally desirable to minimize the frontal area of the submerged drive components in order to reduce drag caused by such components during operation. This presents a difficult problem in high power applications when utilizing a standard perpendicular shaft/bevel gear power transfer arrangement. When the proper gear ratio is selected for the optimum propeller in a high power system, the diameter of the bevel gears increases, thereby causing an increase in the transverse dimension of the portion of the gearcase housing the bevel gears. The resulting increase in gearcase drag certainly detracts from the supposed increase in performance provided by the optimum propeller, and may even result in a decrease in overall performance. That is, the large propeller required for efficiently transferring power through the system requires a gear design furnishing large reduction ratios. Such large reduction ratios are only accomplished by increasing the size of the bevel gears, thereby increasing hydrodynamic drag of the torpedo.
If the diameter of the gearcase torpedo is reduced to a more desirable hydrodynamic size, resulting in a reduction in the size of the gears housed therein, then the gear ratio is likewise reduced to the extent that the propeller no longer operates efficiently.
One solution to the above-described problem is to design a small diameter gearcase in which the propeller runs in a semi-submerged condition. The smaller diameter propellers employed are suitable for the relatively light loads under such circumstances. As a result of such design, however, the low-speed performance of the boat is sacrificed.
Another approach is to employ an inboard system. However, when such a system is used in a high power application, the large diameter propeller required for efficient power transfer results in an increase of the propeller shaft angle in order for the propeller to clear the boat hull. With this arrangement, the vertical component of the propeller thrust increases, thus detracting from the forward component. One solution is to use a smaller diameter propeller, but this results in less efficient power transfer. Alternatively, the engine could be moved forward in the boat, but this encroaches on the interior space in the boat.
The present invention provides a solution to the noted problems encountered in a high power system. The invention contemplates a stern drive system in which the propeller shaft is provided with a sprocket, and is driven by a chain. The chain extends between the propeller shaft and an intermediate shaft located above the waterline during boat operation. The propeller shaft sprocket can be of a relatively small diameter, thereby reducing the frontal area of the gearcase torpedo, and the chain is relatively wide in order to accommodate the high power of the system. The intermediate shaft is disposed substantially parallel to the propeller shaft, and also to an input shaft provided in the upper portion of the gearcase. The input shaft is interconnected with the engine crankshaft so as to be rotatable in response thereto. A drive mechanism is provided between the input shaft and the intermediate shaft for transferring power therebetween. In one embodiment, the intermediate shaft has one or more relatively large diameter sprockets mounted thereto, and the input shaft has corresponding sprockets of relatively small diameter mounted thereto. Chains are provided about the input shaft and intermediate shaft sprockets, and the sprockets are designed to provide a relatively large reduction. This design supplies the large reduction ratios required to efficiently transfer power in a high power system to the large propeller optimally employed in such a system. With the intermediate shaft being located above the waterline during boat operation, the large diameter sprockets mounted thereto are housed within a portion of the gearcase housing which is not submerged during operation, thereby having no effect on the submerged frontal area of the gearcase. A reversing means is preferably provided for selectively imparting rotation to the intermediate shaft in either a first or second rotational direction. The reversing means preferably comprises a drive sprocket mounted to the input shaft and a corresponding driven sprocket mounted to the intermediate shaft, with a chain provided around the drive and driven sprockets. The input shaft also has a drive gear mounted thereto, engageable with a corresponding driven gear provided on the intermediate shaft. Clutch means is provided for selectively coupling either the drive gear or drive sprocket on the input shaft to the driven gear or driven sprocket on the intermediate shaft, thereby resulting in selective rotation of the intermediate shaft in either a first or second rotational direction.
The utilization of a chain drive system in a marine drive is shown in a number of prior art patents. Representative of such systems are U.S. Pat. No. 3,403,655 to Warburton, U.S. Pat. No. 3,951,096 to Dunlap and U.S. Pat. No. 3,088,430 to Champney. In the systems shown in the noted patents, sprockets are connected to the input shaft and to the propeller shaft. A chain is provided between the input shaft sprocket and the propeller shaft sprocket for driving the propeller shaft in response to rotation of the input shaft. However, the referenced patents do not employ an intermediate shaft between the input shaft and the propeller shaft, which allows the large reduction necessary to efficiently transfer high power to a large diameter propeller.